Portable electronic devices, such as radiotelephones, currently use batteries as their main power source. Adapters, such as hands-free adapters, mobile transceiver adapters, cigarette lighter adapters, or wall charger adapters, can be connected to a vehicle cigarette lighter or an electrical outlet to provide an external power source for charging a battery attached to the portable electronic device. Many of these portable electronic devices use internal battery chargers to decrease the size of the adapters and increase convenience to the user.
In one type of internal battery charger, called a series pass charger, a linear switch pass device such as a metal-oxide-semiconductor field-effect transistor (MOSFET) is connected between a regulator and the battery. When a battery is charging, the power dissipated by the pass device is equal to the difference between the input and output voltages of the pass device multiplied by the maximum charging current. When a battery is deeply discharged, the battery voltage, which is the voltage at the output of the pass device, is much less than the regulator voltage, which is the voltage at the input of the pass device. During this condition, the power dissipated by the pass device could exceed maximum power ratings of typical device packages found in portable electronic devices. During a period of high power dissipation by the pass device, excess heat is generated and the overall efficiency of the battery charger is very poor.
Some internal battery chargers use an external tracking regulator physically located in the adapter to limit power dissipation in the charger's pass device. The tracking regulator provides a voltage that is a constant positive offset from the voltage of the battery being charged, thus holding the difference between the input and output voltages of the pass device relatively constant. When charging the battery, a microprocessor creates a control voltage proportional to a desired charging current, which controls the pass device (e.g., the gate of the MOSFET). The actual charging current is measured by a feedback loop that senses a voltage drop across a sense resistor, scales it, and compares it to the control voltage.
Even the use of an external tracking regulator, however, does not sufficiently reduce the power dissipation of an internal battery charger under certain conditions. For example, when the internal battery charger is implemented in a very small radiotelephone, the package of the pass device may be too small to properly dissipate the heat created by the pass device. Using a larger pass device package would make heat dissipation more efficient, but the drawback is that a larger package would make it difficult to fit the internal battery charger into the very small radiotelephone. Thus, it would be advantageous to further limit power dissipation in the internal battery charger to allow a reduction in the size of the pass device.